Wednesday, November 26, 2014

An outline history of phylogenetic trees and networks

This the 300th post on this blog, and so I thought we might have a bit of a summary. Here is the early history of phylogenetic trees and networks as we currently know it. There may, of course, be as yet undetected sources. Details of each of these historical notes (including illustrations) can be found elsewhere in this blog — you can use the search feature in the right side-bar to find them.

Biology

Genealogies as pedigrees (the history of individuals) have a long history. For example, they appear in inscriptions concerning the pharaohs of Ancient Egypt, although these are very imprecise and have caused many headaches for modern scholars. They appear as chains of ancestors and descendants in the Old Testament of the Christian Bible, often contradicting each other and claiming impossible lifespans. Most importantly for modern usage, they were employed in the New Testament to legitimize Jesus as the messiah foretold in the Old Testament. The first known illustration of this appeared in c.400 AD, and it was actually a network, as there were two lineages leading to Jesus (via both Joseph and Mary).

The apparent success of this application (later called the Tree of Jesse, pictures of which started appearing in the 10th century) has meant that both royalty and the nobility have subsequently used pedigrees to assert their own right to be regal and noble. The first known illustration of this is from c.1000 AD, in which Cunigunde of Luxembourg's ancestry was traced in a tree-like manner to include Charlemagne, thus legitimizing her claim to being royal.

Also, up until 1215 AD marriage within seven degrees of separation was not allowed by the christian church, and intestate inheritance applied the same relationship limit. So, a record of blood ties among relatives was often needed; and these started appearing in family bibles, for example. The first recorded tree-like illustrated pedigree was for Lambert of Saint-Omer, which appeared in 1122 AD in his personal copy of his book Liber Floridus.

It seems obvious, then, to also construct genealogies for groups of organisms, which we now call phylogenies (a word coined by Ernst Haeckel in 1866). The Great Chain of Being was for a long time the most popular iconography for relationships, mainly because it neatly tied in with the Christian philosophy of a chain of intellectual ideas, leading from pragmatic earthly concerns and culminating in the idealistic heavens. Humans were, of course, at the head of the chain of earthly beings, and capable of ascending to the heavens.

However, this did not work from a purely observational point of view. Observed pedigrees were not linear, but branched with each generation and often fused again via marriage. Furthermore, biodiversity (the patterns among groups of organisms) also seemed to have multiple relationships. This lead Vitaliano Donati in 1750 (Della Storia Naturale Marina dell' Adriatico) to suggest that:

In addition, the links of the chain are joined in such a way within the links of another chain, that the natural progressions should have to be compared more to a net than to a chain, that net being, so to speak, woven with various threads which show, between them, changing communications, connections, and unions. [from the original Italian]

He was not alone in this thought, although others chose different metaphors. For example, Carl von Linné in 1751 (Philosophia Botanica) wrote this:

All plants show affinities on either side, like territories in a geographical map. [from the original Latin]

Neither author published a reticulating diagram to illustrate their thoughts, although one of Linné's students subsequently produced a version of his ideas in 1792 (Caroli a Linné, Praelectiones in Ordines Naturales Plantarum).

So, it was Georges-Louis Leclerc, Comte de Buffon, who produced the first empirical phylogeny in 1755 (Histoire Naturelle Générale et Particulière, Tome V). This was a network showing the evolutionary origin of domesticated dog breeds. This was followed by Antoine Nicolas Duchesne in 1766 (Histoire Naturelle des Fraisiers), who produced a network showing the evolutionary origin of strawberry cultivars. In both cases the evolutionary process illustrated by the reticulations in the network was hybridization. Note that both of these diagrams refer to within-species genealogies, rather than to relationships between species; and neither author seems to have contemplated the idea of among-species phylogenies.

Thus, in both theory and practice modern phylogenetic metaphors started as networks, not trees. It was Peter Simon Pallas in 1776 (Elenchus Zoophytorum) who first suggested using a tree as a simplified metaphor:

As Donati has already judiciously observed, the works of Nature are not connected in series in a Scale, but cohere in a Net. On the other hand, the whole system of organic bodies may be well represented by the likeness of a tree that immediately from the root divides both the simplest plants and animals, [but they remain] variously contiguous as they advance up the trunk, Animals and Vegetables; [from the origina Latin]

Again, no diagram was forthcoming to illustrate this. It was Jean-Baptiste Pierre Antoine de Monet, Chevalier de Lamarck, who finally produced an empirical phylogeny in 1809 (Philosophie Zoologique). This was a small tree showing the evolutionary relationships among the major groups of animals. However, it represented what we would now call transformational evolution, as Lamarck did not believe in extinction, and thus he showed one group transforming into another. This differed from both Buffon and Duchesne, who were illustrating a process of increasing diversity of groups. It also differed by referring to supra-species relationships.

For the next 50 years, diagrams showing biodiversity relationships illustrated what we now call patterns of affinity, rather than showing historical relationships. These affinity diagrams showed apparent similarities among groups of organisms, without any implication that the relationships were the result of evolutionary history. The majority of these diagrams were networks rather than trees, indicating that groups of organisms had observed similarities with several other groups.

It is Charles Darwin and Alfred Russel Wallace who are credited with introducing, in 1858, the idea that natural selection could be the important process by which new species arise, although the idea of natural selection itself had been "in the air" for more than half a century with respect to within-species variation. (In the case of Patrick Matthew, he had also suggested a role in the origin of new species; 1831, On Naval Timber and Arboriculture; with Critical Notes on Authors who have Recently Treated the Subject of Planting).

As was by now becoming a tradition, neither Darwin nor Wallace (nor Matthew) produced a diagram to illustrate their thoughts. Darwin did draw a theoretical diagram in his subsequent 1859 book (On the Origin of Species by Means of Natural Selection), but he used it to illustrate continuity of evolutionary descent and the processes of extinction and diversification, rather than strictly as representing a phylogeny. His famous "Tree of Life" metaphor had nothing to do with the diagram (it was a Biblical metaphor, to stimulate the imagination of his readers).

The first person to get into print what we could call an empirical diagram representing Darwin's idea was Johann Friedrich Theodor Müller in 1864 (Für Darwin), who drew a small (three-species) tree of amphipods. This was followed by St George Jackson Mivart in 1865 (Contributions towards a more complete knowledge of the axial skeleton in the primates. Proceedings of the Zoological Society of London 33: 545-592). This was a much more extensive diagram illustrating possible evolutionary relationships among primate species (including humans) based solely on their body skeleton.

Confusion between trees and networks reappeared at this time. In particular, Franz Martin Hilgendorf had produced an unpublished PhD thesis in 1863 (Beiträge zur Kenntniß des Süßwasserkalkes von Steinheim) during which he constructed an empirical network of relationships among extinct snail species; but he rejected this because it did not match the Darwinian idea of an evolutionary tree. He later collected more data, and instead published a phylogenetic tree in 1866 (Planorbis multiformis im Steinheimer Süßwasserkalk: ein beispiel von gestaltveränderung im laufe der zeit).

Thus, we last saw an explicit evolutionary network in 1766, referring to with-species variation. The first person to publish an evolutionary network showing relationships among species was apparently Ferdinand Albin Pax in 1888 (Monographische übersicht über die arten der gattung Primula. Botanische Jahrbücher für Systematik, Pflanzengeschichte und Pflanzengeographie 10: 75-241). He produced 14 networks of various primula species, apparently showing affinity relationships, but three of these also illustrate hybridization, which is strictly an evolutionary process.

Anthropology

Genealogies appear in anthropology as well as in biology. Any human creation can be considered to have a history of "descent with modification" if copies are passed from generation to generation (eg. languages, books, tales). For our purposes here, the most important historical developments were in linguistics (languages studies) and in stemmatology (manuscript studies).

Georg Stiernhielm appears to have been the first linguist to draw a genealogy, when he produced a small network of Germanic languages in 1671 (De Linguarum Origine Præfatio, the preface to his edition of Evangelia ab Ulfila Gothorum). This was followed by Félix Gallet in c.1800 (Arbre Généalogique des Langues Mortes et Vivantes), who produced a single broadsheet with a network of Indo-European languages.

Note that, as for biology, the modern metaphors started as networks, not trees. More importantly, note that Stiernhielm's diagram pre-dated Buffon's dog network by more than 80 years — evolutionary ideas were less revolutionary in linguistics than they were in biology.

Darwin explicitly noted a connection between language genealogies and biology genealogies in 1859. However, the first people to get into print what we could call empirical diagrams representing Darwin's idea did so before Darwin published anything on the subject. In 1853 František Ladislav Čelakovský published a tree depicting a history of the Slavic languages (Čtení o Srovnávací Mluvnici Slovanské na Universitě Pražskě), and Auguste Schleicher published one on the development of the Indo-Germanic language family (Die ersten Spaltungen des Indogermanischen Urvolkes. Allgemeine Monatsschrift für Wissenschaft und Literatur 1853: 786-787).

Stemmatology differs from linguistics and biology in first producing a tree rather than a network. Hans Samuel Collin and Carl Johan Schlyter produced this in 1827 (first volume of Corpus Iuris Sueo-Gotorum Antiqui), with a tree of relationships among hand-written copies of documents containing the Medieval laws of Sweden. This was also a tree that represented Darwin's genealogical idea, and so it may be considered to be the first one of that type to be published (ie. 25 years before Čelakovský and Schleicher, and 30 years before Darwin).

This early lead was followed by the first network in 1832, when Friedrich Wilhelm Ritschl's stemma of a book by Thomas Magister (Thomae Magistri sive Theoduli Monachi Ecloga vocum Atticarum) explicitly showed sources of contamination among the manuscript copies — that is, different parts of a manuscript were copied from different sources, rather strict ancestor-descendant copying.

Interestingly, the tree metaphor didn’t endure in anthropology as well as it did in biology. It was quickly replaced by alternative metaphors, such as wave, web, warp & weft, lattice and other continuously reticulating images. Horizontal flow of information has always been seen as a dominant force in anthropological histories.